[llvm] [AArch64][FEAT_CMPBR] Codegen for Armv9.6-a compare-and-branch (PR #116465)

[David Green via llvm-commits]

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@@ -7489,3 +7492,57 @@ bool AArch64DAGToDAGISel::SelectSMETileSlice(SDValue N, unsigned MaxSize,
   Offset = CurDAG->getTargetConstant(0, SDLoc(N), MVT::i64);
   return true;
 }
+
+template <int Bits>
+bool AArch64DAGToDAGISel::SelectCmpBranchUImm6Operand(SDNode *P, SDValue N,
+                                                      SDValue &Imm) {
+  ConstantSDNode *C = dyn_cast<ConstantSDNode>(P->getOperand(1));
+  if (!C)
+    return false;
+
+  AArch64CC::CondCode CC = static_cast<AArch64CC::CondCode>(C->getZExtValue());
+  if (auto *CN = dyn_cast<ConstantSDNode>(N)) {
+    // Check conservatively if the immediate fits the valid range [0, 64).
+    // Immediate variants for GE and HS definitely need to be decremented
+    // when lowering the pseudos later, so an immediate of 1 would become 0.
+    // For the inverse conditions LT and LO we don't know for sure if they
+    // will need a decrement but should the decision be made to reverse the
+    // branch condition, we again end up with the need to decrement.
+    // The same argument holds for LE, LS, GT and HI and possibly
+    // incremented immediates. This can lead to slightly less optimal
+    // codegen, e.g. we never codegen the legal case
+    //    cblt w0, #63, A
+    // because we could end up with the illegal case
+    //    cbge w0, #64, B
+    // should the decision to reverse the branch direction be made. For the
+    // lower bound cases this is no problem since we can express comparisons
+    // against 0 with either tbz/tnbz or using wzr/xzr.
+    uint64_t LowerBound = 0, UpperBound = 64;
+    switch (CC) {
+    case AArch64CC::GE:
+    case AArch64CC::HS:
+    case AArch64CC::LT:
+    case AArch64CC::LO:
+      LowerBound = 1;
+      break;
+    case AArch64CC::LE:
+    case AArch64CC::LS:
+    case AArch64CC::GT:
+    case AArch64CC::HI:
+      UpperBound = 63;
+      break;
+    default:
+      break;
+    }
+
+    if (CN->getAPIntValue().uge(LowerBound) &&
+        CN->getAPIntValue().ult(UpperBound)) {
+      SDLoc DL(N);
+      Imm = CurDAG->getTargetConstant(CN->getZExtValue(), DL,
+                                      Bits == 32 ? MVT::i32 : MVT::i64);
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davemgreen wrote:

This could use Op.getValueType() to get the type, I believe?

https://github.com/llvm/llvm-project/pull/116465